When claiming Foreign Tax Credit Relief (FTCR), the tax credit allowable in the UK is the lower of :
(a) the foreign tax paid (or as allowed in the relevant article of the Double Taxation Agreement) on the income or capital gain (
b) the UK tax liability on the income or capital gain.
If part (or all )of the foreign tax on which you have claimed FTCR is subsequently refunded by the foreign tax authority, you must reduce the amount of FTCR claimed (by amending your self assessment tax return).
Please see Relief for Foreign Tax Paid 2021 (HS263)